What do you mean?
First is that certain elements block scrolling. I use FF primarily for Facebook (since FF allows me to use my – shameless plug – idle-time faker extension [1]). If my scrolling thumb lands on the "Like" button of a post, the screen just stays put. So about 1/3 of my "scroll" gestures are in vain.
Second is that "flinging" motions don't register very well with Firefox. With Chrome, "flinging" sends the web page flying at a predictable speed; with Firefox, it lags for a fraction of a second before scrolling at a random speed which is usually much slower than that in Chrome. It's as if Firefox doesn't register the final thumb flick of a "fling", but Chrome does. Since "flinging" is my primary method of browsing longer web pages, ultimately my thumb has to physically do more work, and gets exhausted more quickly, when using Firefox than Chrome.
A third nitpick is that, while (non-flinging) page scrolling keeps up with my thumb, it does so in a jerky manner, like it can't quite keep up with rendering, or it doesn't see all the touch events. Reminds me of an old PDA. In contrast, scrolling in Chrome is very smooth. This looks ugly but I could live with it.
It is the first to two issues that prevent me from using Firefox on Android for anything but Facebook.
[1] https://addons.mozilla.org/en-US/android/addon/social-networ...